FATTY SUBSTANCES (GENERAL METHODS) 377 the ester number is given by the difference between the acid and saponifi- cation numbers. The ester number has importance especially for the analysis of beeswax. 10. Volatile Acid Number (Reicbert-Meissl Number) By this is meant the number of c.c. of decinormal alkali necessary to neutralise the volatile acids, soluble in water, obtained from 5 grams of a fatty substance under definite conditions. Reichert's original method has been modified in various ways, the modi- fication proposed by Leffmann and Beam and by Polenske being now preferred. In a flat-bottomed, 300 c.c. flask of resistant glass (.4), exactly 5 grams of the fatty substance^ (filtered oil or mol- ten fat) are heated over a small flame with 20 grams of pure glycerine and 2 c.c. of caustic soda solution (100 grams of pure sodium hydro- xide in 100 grams of water) until the liquid froths and becomes clear and homoge- neous (5-8 minutes). The liquid soap thus obtained is allowed to cool to 80-90° and is then shaken with 90 c.c. of distilled water at the same temperature, and heated if necessary, until solution is complete. 50 c.c. of dilute sulphuric acid (25 c.c. of the pure cone, acid to the litre) and 0-6-0-7 gram of roughly powdered pumice are then added and the liquid distilled FlG- 53 into a no c.c. flask. For this distillation the condenser and accessory apparatus have the form and dimensions (m.m.) shown in Fig. 53. The distilling flask is supported on an asbestos card on a ring 6-5 cm. in diameter, and the flame should be such that the no c.c. of distillate are collected in about 19-21 minutes. When the no c.c. of distiUate are collected, the flame is extinguished and the collecting flask replaced by another vessel and left in water at 15° without shaking for 10 minutes. It is then stoppered, inverted four or five times to mix the liquid but not to shake it too much, and filtered through a dry filter 8 cm. in diameter, 100 c.c. of the filtrate being titrated with decinormal potassium hydroxide solution in presence of 3-4 drops of i% m d oil, and a few other oils, have values below 190.